Engine systems may be configured with a boosting device, such as a turbocharger, for providing a boosted aircharge and improving peak power outputs. Therein a turbine is rotated using energy from an exhaust flow, the turbine then driving a compressor which delivers a boosted aircharge to the engine intake. In an effort to meet stringent federal government fuel economy standards, engine systems may also be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. For example, the EGR system may be a low-pressure EGR system (LP-EGR) that recirculates exhaust gas from downstream of an exhaust turbine to upstream of an exhaust compressor. EGR benefits include an increase in engine dilution, decrease in exhaust emissions, and improvements in fuel economy.
The compressor and other intake system components have temperature limitations, to avoid coking and/or thermal stress or damage. Therefore, the EGR is cooled substantially before introduction into the air induction system by a costly EGR cooler (different from the charge air cooler located downstream of the compressor). However, since EGR has relatively large water content (e.g., 8% water by mass for gasoline), the addition of LP-EGR at this substantially lower temperature to the intake at a pre-compressor location increases the risk of condensation at both a compressor inlet as well as a charge air cooler outlet. Specifically, under cold ambient conditions, when the humid EGR is mixed with cold ambient air, water droplets can form. Water droplets impacting the compressor blades which are rotating at high speeds (e.g., 200,000 rpm or above) can cause damage to the blades. In addition, since the ingested water slows the rate of combustion, the introduction of water into the engine can increase the likelihood of misfire events.
To address these issues, engine control systems may employ various approaches to limit the condensation. One example approach is shown by Rimnac et al. in U.S. Pat. No. 6,681,171. Therein, if conditions favoring EGR condensation are determined, intake manifold temperatures are increased by redirecting some or all of the EGR flow to avoid the EGR cooler. Additionally, some or all of the charge air is redirected from an outlet of the compressor to the engine intake, while bypassing the charge air cooler. As a result, a higher effective intake air temperature is provided which reduces the likelihood of condensation.
However, the inventors herein have identified potential issues with such an approach. Since the approach of '171 does not actively estimate humidity at the engine intake, there may be ambient temperature and humidity conditions where even with the redirection of the charge air, condensation occurs at the engine intake, degrading, boosted engine performance. It also does not address the compressor temperature limitations.
In one example, some of the above issues may be addressed by a method for an engine comprising: mixing compressor recirculation flow from downstream of a charge air cooler with exhaust gas, and delivering the mixture to a compressor inlet. In this way, hot EGR that has not been cooled by an EGR cooler can be mixed with cooled compressor recirculation flow to improve temperature and relative humidity management of the compressor inlet air.
As one example, a boosted engine system may include a first passage for recirculating compressed air from downstream of a charge air cooler (and upstream of an intake throttle) to a compressor inlet via a compressor recirculation valve. In this way, compressor recirculation flow that has been cooled upon passage through the charge air cooler can be delivered to the compressor inlet. The engine may further include a second passage for recirculating exhaust gas from downstream of an exhaust turbine to the compressor inlet, unobstructed, via an EGR valve. In this way, hot EGR that has not passed through an intercooler or any other secondary cooling device can be delivered to the compressor inlet. The first and second passages may join at a location upstream of the compressor and downstream of both the valves. By adjusting a position of the first and second valves, cooled compressor recirculation flow may be mixed with the hot EGR to provide a temperature-controlled mixture to the compressor inlet. For example, the composition of the mixture may be adjusted to increase the amount of hot EGR and decrease the amount of compressor recirculation flow when the compressor inlet temperature is low. As another example, the composition of the mixture may be adjusted to decrease the amount of hot EGR and increase the amount of compressor recirculation flow when the compressor outlet temperature is high, or when there is a possibility of compressor surge.
In this way, compressor temperature management is improved. By not requiring cooled EGR, the need for a dedicated EGR cooler is reduced. For example, the cooler may be dramatically downsized or removed from the design, providing component reduction benefits. By using cooled compressor recirculation flow to cool the EGR, a relative humidity and temperature at the compressor inlet, as well at a compressor outlet, can be tuned more precisely. In particular, the compressor inlet and outlet temperatures can be maintained within a range that improves turbocharger performance. By reducing the formation of condensation at the compressor inlet, issues related to the ingestion of condensate (such as misfire, degradation of compressor blades, and engine stall) can be better addressed. Further, use of cooled compressor recirculation flow can concurrently improve the compressor's margin to surge, while also reducing thermal amplification effects that can occur with the use of warm compressor recirculation flow. Overall, boosted engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.